The mechanisms underlying innate resistance of Biomphalaria glabrata snails to infection with Schistosoma mansoni are not known, and specifically the role of the hematopoietic tissue, the so-called amoebocyte-producing organ (APO), in such resistance is unclear. A possible approach to this problem is suggested by the recent demonstration of adoptive transfer of resistance with allografts of the APO from genetically resistant donors. This project seeks to elucidate the mechanism for such transferred resistance, i.e., whether it is due to production of hemocytes (hemocytic chimerism) and/or secretion of soluble "resistance factors" by the implant, and to determine whether additional immunocompetent tissues are present in snails. APO allograft recipients will be examined for increased mitotic activity following exposure to schistosome miracidia, increased numbers of circulating hemocytes, presence of donor hemocyte DNA, and the presence of soluble factors normally found only in resistant plasma, i.e., yeast-opsonizing and sporocyst-agglutinating activity. Additional tissues and organs of resistant snails will also be studied for their ability to transfer schistosome resistance. Finally, several variables that may affect transferred resistance will be examined: age of the donor, size of the implant, and state of activation of the implant. The potential significance of this research is that it may help identify specific organs, tissues, and cells involved in innate resistance to schistosome infection. Furthermore, should hemocytic chimerism be demonstrated, the hematopoietic function of the APO will have been verified.